For people to have a sinful nature means that they only able to sin. They cannot serve God in any way. Adam was the first man. Adam sinned, and now everyone born after Adam was born with a sinful nature (see: Romans 5:12, 14, 18). Every person sins because he has a sinful nature. Scholars sometimes use the words “the fall of man” and “original sin” to talk about how Adam’s sin affects everyone born after him.
In the New Testament, the sinful nature is called a ruler or slave owner. That is, this sin nature controls people (see: Romans 6:6, 14, 17; 7:8-11, 14). In Greek, the word “flesh” (σάρξ/g4561) also means the sinful nature. “Earthly nature” also means “sinful nature” (see: Colossians 3:5).
Because of the sinful nature, humans are not able to do anything to honor God (Romans 7:21-23; 8:7). God will judge all people. Those who reject God will be sent to hell because of their sinful nature (see: Ephesians 2:3). However, God sent Jesus into the world. When people believe in Jesus, the Holy Spirit comes to live in them in some way. It is the Holy Spirit who enables people to do the things that honor God (see: Romans 8:1-17).